Vasculogenesis involves the de novo formation of blood vessels from endothelial cell precursors or angioblasts. The first vascular structures in the embryo are formed by vasculogenesis. Angiogenesis involves the development of capillaries from existing blood vessels, and is the principle mechanism by which organs, such as the brain and the kidney are vascularized. While vasculogenesis is restricted to embryonic development, angiogenesis can occur in the adult, for example during pregnancy, the female cycle, or wound healing.
One major regulator of angiogenesis and vasculogenesis in both embryonic development and some angiogenic-dependent diseases is vascular endothelial growth factor (VEGF; also called vascular permeability factor, VPF). VEGF represents a family of isoforms of mitogens existing in homodimeric forms due to alternative RNA splicing. The VEGF isoforms are highly specific for vascular endothelial cells (for reviews, see: Farrara et al. Endocr. Rev. 1992, 13, 18; Neufield et al. FASEB J. 1999, 13, 9).
VEGF expression is induced by hypoxia (Shweiki et al. Nature 1992, 359, 843), as well as by a variety of cytokines and growth factors, such as interleukin-1, interleukin-6, epidermal growth factor and transforming growth factor-α and -β.
To date VEGF and the VEGF family members have been reported to bind to one or more of three transmembrane receptor tyrosine kinases (Mustonen et al. J. Cell Biol., 1995, 129, 895), VEGF receptor-1 (also known as flt-1 (fms-like tyrosine kinase-1)), VEGFR-2 (also known as kinase insert domain containing receptor (KDR); the murine analogue of KDR is known as fetal liver kinase-1 (flk-1)), and VEGFR-3 (also known as flt-4). KDR and flt-1 have been shown to have different signal transduction properties (Waltenberger et al. J. Biol. Chem. 1994, 269, 26988); Park et al. Oncogene 1995, 10, 135). Thus, KDR undergoes strong ligand-dependant tyrosine phosphorylation in intact cells, whereas flt-1 displays a weak response. Thus, binding to KDR is a critical requirement for induction of the full spectrum of VEGF-mediated biological responses.
In vivo, VEGF plays a central role in vasculogenesis, and induces angiogenesis and permeabilization of blood vessels. Deregulated VEGF expression contributes to the development of a number of diseases that are characterized by abnormal angiogenesis and/or hyperpermeability processes. Regulation of the VEGF-mediated signal transduction cascade will therefore provide a useful mode for control of abnormal angiogenesis and/or hyperpermeability processes.
Angiogenesis is regarded as an absolute prerequisite for growth of tumors beyond about 1-2 mm. Oxygen and nutrients may be supplied to cells in tumor smaller than this limit through diffusion. However, every tumor is dependent on angiogenesis for continued growth after it has reached a certain size. Tumorigenic cells within hypoxic regions of tumors respond by stimulation of VEGF production, which triggers activation of quiescent endothelial cells to stimulate new blood vessel formation. (Shweiki et al. Proc. Nat'l. Acad. Set., 1995, 92, 768). In addition, VEGF production in tumor regions where there is no angiogenesis may proceed through the ras signal transduction pathway (Grugel et al. J. Biol. Chem., 1995, 270, 25915; Rak et al. Cancer Res. 1995, 55, 4575). In situ hybridization studies have demonstrated VEGF mRNA is strongly upregulated in a wide variety of human tumors, including lung (Mattern et al. Br. J. Cancer 1996, 73, 931), thyroid (Viglietto et al. Oncogene 1995, 11, 1569), breast (Brown et al. Human Pathol. 1995, 26, 86), gastrointestinal tract (Brown et al. Cancer Res. 1993, 53, 4727; Suzuki et al. Cancer Res. 1996, 56, 3004), kidney and bladder (Brown et al. Am. J. Pathol. 1993, 1431, 1255), ovary (Olson et al. Cancer Res. 1994, 54, 1255), and cervical (Guidi et al. J. Nat'l Cancer Inst. 1995, 87, 12137) carcinomas, as well as angiosarcoma (Hashimoto et al. Lab. Invest. 1995, 73, 859) and several intracranial tumors (Plate et al. Nature 1992, 359, 845; Phillips et al. Int. J. Oncol. 1993, 2, 913; Berkman et al. J. Clin. Invest., 1993, 91, 153). Neutralizing monoclonal antibodies to KDR have been shown to be efficacious in blocking tumor angiogenesis (Kim et al. Nature 1993, 362, 841; Rockwell et al. Mol. Cell. Differ. 1995, 3, 315).
Over expression of VEGF, for example under conditions of extreme hypoxia, can lead to intraocular angiogenesis, resulting in hyperproliferation of blood vessels, leading eventually to blindness. Such a cascade of events has been observed for a number of retinopathies, including diabetic retinopathy, ischemic retinal-vein occlusion, retinopathy of prematurity (Aiello et al. New Engl. J. Med. 1994, 331, 1480; Peer et al. Lab. Invest. 1995, 72, 638), and age-related macular degeneration (AMD; see, Lopez et al. Invest. Opththalmol. Vis. Sci. 1996, 37, 855).
In rheumatoid arthritis (RA), the in-growth of vascular pannus may be mediated by production of angiogenic factors. Levels of immunoreactive VEGF are high in the synovial fluid of RA patients, while VEGF levels were low in the synovial fluid of patients with other forms of arthritis of with degenerative joint disease (Koch et al. J. Immunol. 1994; 152, 4149). The angiogenesis inhibitor AGM-170 has been shown to prevent neovascularization of the joint in the rat collagen arthritis model (Peacock et al. J. Exper. Med. 1992, 175, 1135).
Increased VEGF expression has also been shown in psoriatic skin, as well as bullous disorders associated with subepidermal blister formation, such as bullous pemphigoid, erythema multiforme, and dermatitis herpetiformis (Brown et al. J. Invest. Dermatol. 1995, 104, 744).
Because inhibition of KDR leads to inhibition of VEGF-mediated angiogenesis and permeabilization, KDR inhibitors will be useful in treatment of diseases characterized by abnormal angiogenesis and/or hyperpermeability processes, including the above listed diseases